As is known, many liquid or pourable food products, such as fruit juice, UHT (ultra-high-temperature treated) milk, wine, tomato sauce, etc., are sold in packages made of sterilized packaging material.
A typical example is the parallelepiped-shaped package for liquid or pourable food products known as Tetra Brik Aseptic (registered trademark), which is made by folding and sealing laminated strip packaging material. The packaging material has a multilayer structure comprising a base layer, e.g. of paper, covered on both sides with layers of heat-seal plastic material, e.g. polyethylene. In the case of aseptic packages for long-storage products, such as UHT milk, the packaging material also comprises a layer of oxygen-barrier material, e.g. an aluminium foil, which is superimposed on a layer of heat-seal plastic material, and is in turn covered with another layer of heat-seal plastic material forming the inner face of the package eventually contacting the food product.
Packages of this sort are normally produced on fully automatic packaging units, on which a continuous tube is formed from the web-fed packaging material; the web of packaging material is sterilized on the packaging unit, e.g. by applying a chemical sterilizing agent, such as a hydrogen peroxide solution, which, once sterilization is completed, is removed from the surfaces of the packaging material, e.g. evaporated by heating; the web so sterilized is then maintained in a closed, sterile environment, and is folded and sealed longitudinally to form a tube, which is fed along a vertical advancing direction.
In order to complete the forming operations, the tube is filled with the sterilized or sterile-processed food product, and is sealed and subsequently cut along equally spaced transversal cross sections.
Pillow packs are so obtained, which have a longitudinal sealing band and a pair of top and bottom transversal sealing bands.
Packaging units are known, as described for example in European Patent EP-B-0887265, which comprise two chain conveyors defining respective endless paths.
In greater detail, the first conveyor is fitted with a plurality of first jaws, each comprising a respective sealing element and a respective extractable cutting element.
The second conveyor is fitted with a plurality of second jaws, each associated to a relative first jaw and comprising a respective counter-sealing element and a respective seat.
In particular, each sealing element is a heating element and corresponding counter-sealing element is made of elastomeric material, which provides the necessary mechanical support to grip the tube to the required pressure.
The paths of the chain conveyors comprise:                respective operative branches substantially facing and parallel to each other, and between which the tube of packaging material is fed, so that the first jaws cooperate with the second jaws on the other conveyor to grip the tube at a number of successive cross sections and to heat-seal the tube and cut the packs; and        respective return branches, along which the first jaws and the second jaws are spaced apart from the tube.        
In greater detail, the first jaws and the corresponding second jaws at first move away from each other and then move towards each other, as they move along return branches downstream of the operative branches with respect to the advancing direction of the chain conveyors.
As each first jaw cooperates with the corresponding second jaw along the operative branches, the respective sealing element cooperates with the respective counter-sealing element to heat-seal the tube at a corresponding transversal section.
Furthermore, as each first jaw cooperates with the corresponding second jaw, the respective cutting element is extracted to cut the formed sealed packs at the transversal section, so as to form corresponding sealed packages.
Each first jaw and second jaw also comprises respective forming half-shells, which are hinged with respect to the relative sealing element or the counter-sealing element, so as to control the volume of the relative packs in formation.
The half-shells of each first jaw and of the corresponding second jaw move cyclically between:                an open position, in which they are detached from the tube; and        a closed position, in which they contact the tube and fold the portion of the tube between two consecutive sealing sections to define and control the volume of the packs being formed.        
In this way, as the sealing element of each first jaw seals the pack in formation, the half-shells of the same first jaw and of the corresponding second jaw control the volume of the pack in formation.
Furthermore, each first jaw comprises a pair of lateral fingers, which are arranged immediately above the sealing element.
The fingers are adapted to pull down the tube when the half-shells are closing on the tube, so as to arrange the tube in a desired position with respect to the sealing element of the same first jaw.
In this way, it is ensured that the packs are transversally sealed in the desired position with respect to the images repeatedly printed on the packaging material forming the tube. The images must be arranged in given position with respect to the transversal sealed sections of the formed packs.
Furthermore, it is important to consider that the pack comprises a substantially parallelepiped main portion and a pair of opposite trapezoidal fins projecting from the main portion. Each fin comprises a pair of opposite triangular flaps, which laterally extends beyond the main portion on respective opposite lateral sides.
In order to correctly form the packs, the fingers press and fold the triangular flaps orthogonally to the axis of the tube and on the sealing element of the relative first jaw, when the latter cooperates with relative second jaw.
In other words, as the tube is constrained by the first jaw and second jaw cooperating with one another the fingers fold the triangular flaps, thus completing the formation of the relative pack.
More specifically, the half-shells may be spring-loaded by respective springs into the respective open position, and have respective rollers, which cooperate with respective cams designed to move the half-shells into the respective closed position by the time the forming assembly reaches a predetermined position as it moves down.
Each half-shell has a C-shaped cross section, and comprises, integrally: a main wall, and two parallel lateral flaps projecting towards the axis of the tube of packaging material from respective opposite end edges of the main wall.
In the closed position, the main walls are located on opposite sides of the tube axis, are parallel to each other, and cooperate with respective first portions of the tube.
In the closed position, the flaps of one half-shell cooperate with respective second portions of the tube to completely control the volume of the pack being formed, and, on the opposite side to the relative main wall, face corresponding flaps of the other half-shell.
Finally, the packaging unit comprises a pair of fixed cam assemblies arranged on respective sides of the tube.
The cam assemblies cooperate in rolling manner with respective rollers carried by the first jaws and the second jaws.
The profiles of the cam assemblies are so designed to move the first jaws and the second jaws along the respective return branches and operative branches, and to ensure the smoothest transition between the various positions assumed by the first jaws and the second jaws, so as to prevent undesired stress on the materials.
Though performing excellently on the whole, the packaging units of the type described still leave room for further improvement.
In particular, it has been proposed, in the embodiment shown in FIG. 17 of WO00/64741, to replace the conveyor chains with:                a first track and a second track, which are arranged on respective opposite lateral sides of the tube; and        a plurality of first carriages self-movable on the first track and a plurality of respective second carriages movable on the second track.        
In particular, the first carriages are movable independently of one another along the first track and the second carriages are movable independently of one another along the second track.
Each first carriage is fitted with a relative sealing element and a relative half-shell, while each corresponding second carriage is fitted with a respective counter-sealing element and with a relative half-shell.
Being the first carriage and the respective second carriage movable independently from one another, it is no longer necessary to provide the cumbersome chain conveyors.
However, the packaging unit shown in FIG. 17 of WO00/64741 leaves room for improvement, especially as regards to the precise and repeatable sealing of the pillow packs in the desired position.
As a matter of fact, the packaging unit shown in FIG. 17 of WO00/64741 fails to disclose any solution for the folding of the triangular top flaps of the packs in formation.
A need is therefore felt within the industry to improve the precision and the repeatibility with which the tube is arranged in the desired position with respect to the sealing element and counter-sealing element.
Furthermore, the solution shown in FIG. 17 of WO00/64741 leaves room for improvement, especially as regard to the possibility of forming sealed packs having different height and the diameter varying in a narrow range, without wholly re-designing the packaging unit.
As a matter of fact, in order to adjust the height of the formed sealed packs, it is necessary to replace the whole first carriages and second carriages with differently-sized ones that fit with the new packs.
A need is felt within the industry to improve the flexibility of the packaging units, with respect to the possibility of forming sealed packs having different heights and with diameter varying in a narrow range.